1. Field of the Invention
The present invention relates generally to an overcurrent tripping unit for a circuit breaker, and more specifically, it relates to an overcurrent tripping unit for a circuit breaker in which the tripping unit operates to break a main circuit when a current flowing in the main circuit exceeds a predetermined set value and a predetermined time condition is satisfied.
2. Description of the Prior Art
FIG. 1 is an electrical circuit diagram of a current detecting portion in a conventional overcurrent tripping unit. Referring to FIG. 1, the structure of a conventional overcurrent tripping unit will be described using a three-pole circuit breaker as an example. A circuit breaker CB is inserted in each phase of the main circuit 1, and current transformers CT1 to CT3 are provided at the output side of the respective circuit breakers CBs. One end of the current transformer CT1 is connected to an anode of a diode D1 and to a cathode of a diode D2; one end of the current transformer CT2 is connected to an anode of a diode D3 and to a cathode of a diode D4; and one end of the current transformer CT3 is connected to an anode of a diode D5 and to a cathode of a diode D6.
The other ends of the current transformers CT1 to CT3 are commonly connected to one end of a primary coil of a current transformer CT4 for detecting the ground fault current. The other end of the primary coil of the current transformer CT4 is connected to an anode of a diode D7 and to a cathode of a diode D8. The current transformer CT4 is to detect the ground fault current. The cathode of each of the diodes D1, D3, D5 and D7 is connected to an IN2 input end of a control circuit 2 as well as to an IN1 input end of the control circuit 2 through a resistance R1. The anode of each of the diodes D2, D4, D6 and D8 is connected to an IN4 input end of the control circuit 2.
The secondary outputs of the current transformer CT4 are connected to the diodes D9 to D12 to be full-wave rectified, with the full-wave rectified output being connected to an IN3 input end of the control circuit as well as the IN1 input end of the control circuit 2 through a resistance R2. A smoothing capacitor C1 and a voltage regulating diode D13 are connected in parallel between the IN1 input end and the IN4 input end of the control circuit 2. The combination of a resistance, a voltage regulating diode and a transistor may be used instead of the voltage regulating diode D13 to form a constant voltage circuit.
A semiconductor trip circuit such as disclosed in U.S. Pat. No. 4,380,785 is contained in the control circuit 2. The semiconductor trip circuit has a delayed releasing characteristic, that is the circuit breaker CB is broken within a predetermined time period when a current flowing in the main circuit 1 exceeds a predetermined permissible current, and an instant releasing characteristic, that is, the circuit breaker CB is broken instantly when the current flowing in the main circuit 1 exceeds a higher predetermined value.
FIGS. 2A and 2B are diagrams of waveforms showing the waveforms of the current flowing in the main circuit of FIG. 1 and the rectified and composite secondary currents of this current.
The operation of the current detecting portion of the overcurrent tripping unit shown in FIG. 1 will be described with reference to FIGS. 2A and 2B. The phases of the currents i.sub.A, i.sub.B and i.sub.C respectively flowing in the phases A, B and C of the main circuit 1 are shifted from each other by 120.degree. as shown in FIG. 2A. The currents of the respective phases detected by the current transformers CT1 to CT3 are full-wave rectified by the diodes D1 to D6 and, consequently, a voltage drop such as shown in FIG. 2B is generated at both ends of the resistance R1. More specifically, the voltage drop generated at the resistance R1 of FIG. 1 is consisted by the sum of the forward direction component of the CT secondary current of the currents i.sub.A, i.sub.B and i.sub.C of the respective phases. If the waveform of the current is perfectly sinusoidal, an effective value can be obtained by setting the voltage peak value at 1/.sqroot.2. On this occasion, the current flowing in the secondary side of the current transformer CT4 is 0 as long as there is no accidental grounding.
However, recently a number of power electronics devices such as an inverter and a thyristor control unit are used as loads of the main circuit 1, so that the waveform of the current has been deformed. Therefore, the control can not be carried out based on the current value corresponding to the effective value of the respective phase currents i.sub.A, i.sub.B and i.sub.C when the current peak only is detected in the above described manner. In order to eliminate the disadvantage, the independent detection of the currents i.sub.A, i.sub.B and i.sub.C of respective phase becomes necessary.
FIG. 3 is an electrical circuit diagram of the current detecting portion of an overcurrent tripping unit in which the current of each phase can be detected. FIGS. 4A to 4D are waveforms of respective portions shown in FIG. 3.
Referring to FIG. 3, as in the above described FIG. 1, a circuit breaker CB and current transformers CT1 to CT3 are inserted in the main circuit 1. One end k.sub.A of the current transformer CT1 is connected to an anode of a diode D1 and to a cathode of a diode D2, while the other end l.sub.A is connected to an anode of a diode D14 and to a cathode of a diode D15. The output of the current transformer CT1 is full-wave rectified by these diodes D1, D2, D14 and D15 to be applied to a control circuit 2 through a resistance R3.
One end k.sub.B of the current transformer CT2 is connected to an anode of a diode D3 and to a cathode of a diode D4, while the other end l.sub.B is connected to an anode of the diode D16 and to a cathode of the diode D17. The output of the current transformer CT2 is full-wave rectified by these diodes D3, D4, D16 and D17 to be applied to the control circuit 2 through a resistance R4. Similarly, one end k.sub.C of the current transformer CT3 is connected to an anode of a diode D5 and to a cathode of a diode D6, while the other end l.sub.C is connected to an anode of a diode D18 and to a cathode of a diode D19. The output of the current transformer CT3 is full-wave rectified by these diodes D5, D6, D18 and D19 to be applied to the control circuit 2 through a resistance R5.
By the above described structure, when respective phase currents i.sub.A, i.sub.B and i.sub.C flow in the respective phases of A, B and C as shown in FIG. 4A, then voltage drops such as shown in FIGS. 4B to 4D are generated at the resistances R3, R4 and R5.
Now, a problem in the circuit shown in FIG. 3 is the detection of the ground fault current. In the example shown in FIG. 1, the other ends of the current transformers CT1 to CT3 are respectively short-circuited, while in the example shown in FIG. 3, the other ends l.sub.A to l.sub.C of the current transformers CT1 to CT3 are not short-circuited. If the other ends l.sub.A to l.sub.C of the current transformers CT1 to CT3 are short-circuited, the voltage drop at the resistances R3, R4 and R5 will not have the waveforms shown in FIGS. 4B to 4D.
As for the path in which the current flowing out from the other end l.sub.A of the current transformer CT1 returns to one end k.sub.A of the current transformer CT1 in the A phase, in FIG. 3, the current flows from the other end l.sub.A of the current transformer CT1.fwdarw.diode D14.fwdarw.resistance R3.fwdarw.control circuit 2.fwdarw.diode D2 to the one end k.sub.A of the current transformer CT1, with all currents passing through the resistance R3, therefore there is no problem. However, if the other ends l.sub.A to l.sub.C of the current transformers CT1 to CT3 are short circuited two by-passes are formed beside the above path, namely, the other end l.sub.A of the current transformer CT1.fwdarw.diode D16 through the other end l.sub.B of the current transformer 2.fwdarw.resistance R4.fwdarw.control circuit 2.fwdarw.diode D2.fwdarw.one end k.sub.A of the current transformer CT1, and, the other end l.sub.A of the current transformer CT1.fwdarw.diode D18 through the other end l.sub.C of the current transformer CT3.fwdarw.resistance R5.fwdarw.control circuit 2.fwdarw.diode D2.fwdarw.one end k.sub.A of the current transformer CT1. Consequently, the current also flows through the resistances R4 and R5, so that the voltage drop at the resistance R3 is not proportional to the current i.sub.A flowing in the A phase. For this reason, in the example shown in FIG. 3, the current transformer for detecting the ground fault current such as shown in the above FIG. 1 can not be connected. The configuration shown in FIG. 5 must be employed to incorporate the current transformer for detecting the ground fault current.
FIG. 5 is an electric circuit diagram showing one example of a current detecting portion in an overcurrent tripping unit having a current transformer for detecting the ground fault current incorporated therein. FIG. 6 shows the relation between the current flowing in the main circuit and the output current of the current transformer in FIG. 5.
The circuit shown in FIG. 5 is the same as that shown in FIG. 3 except the following points. Namely, the other end l.sub.A of the current transformer CT1 is connected to the anode of the diode D14 and to the cathode of the diode D15 through a primary coil m.sub.A of the current transformer CT5 for detecting the ground fault current. The end l.sub.B of the current transformer CT2 is connected to the anode of the diode D16 and to the cathode of the diode D17 through the current transformer CT5 for detecting the ground fault current. In addition, the other end l.sub.C of the current transformer CT3 is connected to the anode of the diode D18 and to the cathode of the diode D19 through a primary coil m.sub.C of the current transformer CT5 for detecting the ground fault current. The secondary coil of the current transformer 5 for detecting the ground fault current is connected to a full-wave rectifying circuit comprising diodes D9 to D12, with the full-wave rectified voltage applied to the control circuit 2 through the resistance R2.
In the example shown in FIG. 5, the voltage drop corresponding to the currents flowing through the respective phases A to C are generated at the resistances R3 to R5 and the ground fault current can be detected by the current transformer CT5. However, three coils are required as the primary coils of the current transformer CT5 for detecting the ground fault current. In the four-pole circuit breaker which breaks not only the voltage lines but also the neutral line, four coils are required, enlarging the size of the current transformer CT5 for detecting the ground fault current.
In the current transformer CT4 for detecting the ground fault current shown in FIG. 1, no current flows through the primary coil unless an accidental grounding happens. However, in the current transformer CT5 for detecting the ground fault current shown in FIG. 5, the secondary currents of the current transformers CT1 to CT3 are always flowing through the primary coil of the current transformer CT5. Generally, the set value of the ground fault current is about 10 to 40% of the rated value of the currents i.sub.A to i.sub.C flowing through the main circuit 1 and, when the ground protection circuit is activated, the circuit breaker CB must be operated in several 100 m sec. Therefore, if there is no ground fault current flowing as shown in FIG. 1, the sectional area of the strand of the primary coil of the current transformer CT4 for detecting the ground fault current is permissible to be smaller than the sectional area of the strand of the secondary coils of the current transformers CT1 to CT3 in the main circuit. In other words, thin strands can be used. However, if the secondary current of the main circuit 1 always flows to the primary coils m.sub.A to m.sub.C of the current transformer CT5 for detecting the ground fault current, the sectional area of the strand of the primary coils m.sub.A to m.sub.C of the current transformer CT5 for detecting the ground fault current must be equal to the sectional area of the strand of the secondary coils of the current transformers CT1 to CT3 in the main circuit. For this reason, besides the increase of the number of the primary coils m.sub.A to m.sub.C such as described in the foregoing, there is a disadvantage that each coil becomes large.
In addition, even if there is no accidental grounding, the current constantly flows through the primary coils m.sub.A to m.sub.C of the current transformer CT5 for detecting the ground fault current, so that, viewed from the current transformers CT1 to CT3 in the main circuit 1, the primary coils m.sub.A to m.sub.C of the current transformer CT5 for detecting the ground fault current become a burden even when there is no accidental grounding. More specifically, even if there is no accidental grounding, the outputs of the current transformers CT1 to CT3 are as shown in FIG. 6.
Namely, assuming that the ideal curve of the relation between the main circuit current and the outputs of the current transformers CT1 to CT3 obtained from the calculation is represented by a, it becomes as shown by the curve b when current transformer CT5 for detecting the ground fault current is not provided (the case shown in FIG. 3) and it becomes as represented by the curve c when the current transformer CT5 for detecting the ground fault current is provided, owing to the current transformer CT5. Due to the characteristics shown in FIG. 6, the error to the instantaneous current setting (usually four times to sixteen times of the rated value of the current transformers CT 1 to CT3) differs dependent on whether the ground fault protection is provided or not. Consequently, circuits of different designs are required for each specification. In addition, if the instantaneous current setting becomes near fifteen times the rated value of the current transformer, the outputs of the current transformer CT1 to CT3 become saturated for the characteristics, so that the set precision itself is degraded.
Meanwhile, in FIG. 1, no current flows through the current transformer CT4 for detecting the ground fault current unless an accidental grounding occurs. Namely, the current transformer CT4 for detecting the ground fault current is not a burden for the current transformers CT1 to CT3 of the main circuit, so that it has no such problem as described above.
FIGS. 7 and 8 are electric circuit diagrams showing other examples of the current detecting portions in a conventional overcurrent releasing apparatus.
In the prior art shown in FIGS. 3 and 5, the current of each phase is independently detected. In the example shown in FIG. 7, the current of each phase is not independently detected but the detected voltage of each of the current transformers CT1 to CT3 are full-wave rectified by bridge rectifying diodes D21 to D23, with the respective rectified outputs being overlapped with each other to be applied to the control circuit 2. The relation between the current flowing through the main circuit 1 and the voltage drop at the resistance R1 in the example of FIG. 7 is as the same as in the circuit shown in FIG. 1.
In order to detect the ground fault current in the circuit shown in FIG. 7, the configuration of FIG. 8 is required. Namely, the connection between the primary coil of the current transformer CT5 for detecting the ground fault current and respective current transformers CT1 to CT3 as the same as shown in FIG. 5, and the secondary coil is connected to a bridge rectifying diode D24. In this example also, three coils are required as the primary coils of the current transformer CT5 for detecting the ground fault current, enlarging the current transformer CT5 for detecting the ground fault current.